I’ve actually dealt with a similar situation in one of my projects. The key is to request the right scopes during the initial OAuth flow.
When you set up your Auth.js Google provider, you can specify additional scopes beyond just the basic profile info. You’ll want to include the Gmail API scopes you need, like ‘https://www.googleapis.com/auth/gmail.readonly’ for read-only access.
Once you’ve got the right scopes, you can use the access token from the Auth.js session to make Gmail API requests. No need for a second login.
One gotcha to watch out for: make sure your Google Cloud Console project has the Gmail API enabled. I spent hours debugging before I realized I’d forgotten that step!
Hope this helps point you in the right direction. Let me know if you run into any other snags along the way.